The present invention relates to a process for the preparation of organophosphorus compounds useful as agricultural and horticultural bactericidal and insecticidal agents. More particularly, the present invention relates to an industrially advantageous process for the preparation of organophosphorus compounds, i.e., O,O-dialkyl-S-benzyl thiophosphates, in high purity and efficiency.
Known hitherto as a method for preparing O,O-dialkyl-S-benzyl thiophosphates is the following two-step process. In the first step, a diester of phosphorous acid is reacted according to the following reaction formulas with sulfur and a metal carbonate or ammonium carbonate to prepare a metal salt or ammonium salt of a monothiophosphoric acid diester (Japanese Patent Publn. No. 18367/63). ##STR1##
The resultant metal or ammonium salt of a monothiophosphoric acid diester is then reacted according to the following reaction formulas with a benzyl halide to prepare the desired O,O-dialkyl-S-benzyl thiophosphate [Japanese Patent Publn. No. 2968/57; a Japanese book entitled "Chemistry of Agricultural Agents" (a book in industrial chemistry series) compiled by the Chemical Society of Japan and published by Dainihon Tosho K. K., pages 61-62]. ##STR2##
However, one of the largest drawbacks in the known prior process is that the formation of carbon dioxide as by-product is unavoidable, as is evident from the above reaction formulas, during the reaction for preparing a metal or ammonium salt of a monothiophosphoric acid diester. When the reaction is carried out according to the known prior process, a large amount of carbon dioxide is violently evolved as the reaction proceeds so that a bumping phenomenon often takes place during the reaction whereby the reaction solvent and the starting materials are entrained in the evolved carbon dioxide and escape from the reaction system, thus incurring considerable loss of the reaction solvent and the starting materials used. As it is extremely difficult to control evolution of carbon dioxide by adjustment of the reaction temperature, the known prior process is not satisfactory as an industrially applicable process.
Further detrimental drawback in the prior process is that the end product, i.e., O,O-dialkyl-S-benzyl thiophosphate, is contaminated with impurities including sulfur used as the starting material. Accordingly, it is impossible to prepare the desired O,O-dialkyl-S-benzyl thiophosphate in a high purity without necessity of further complicate operations for eliminating the sulfur contaminant.
Many of these drawbacks result from the use of a metal carbonate or ammonium carbonate as one of the starting materials. Thus, the use of a metal hydroxide was proposed in this case to overcome the drawbacks. However, the use of a metal hydroxide easily tends to permit hydrolysis of a diester of phosphorous acid used as the main starting material. In addition, a side reaction tends to occur between a metal hydroxide and sulfur which is another starting material, thus permitting the formation of thiosulfates. For these reasons, the use of a metal hydroxide fails to attain the purpose aimed at. Especially noteworthy is that as alkalinity of a metal hydroxide used becomes stronger, hydrolysis of the diester of phosphorous acid and the side reaction between a metal hydroxide and sulfur will be promoted.
According to our study, it has been found that when the reaction is conducted by using a hydroxide of an alkali metal such as sodium or potassium, the following reactions take place in the prior process: ##STR3## When water exists in the reaction system, the diester of phosphorous acid is rapidly hydrolyzed.
From these facts, it has been believed that alkali metal salts of monothiophosphoric acid diesters cannot be prepared from diesters of phosphoric acid, sulfur and alkali metal hydroxide and that O,O-dialkyl-S-benzyl thiophosphates aimed at cannot be prepared in a good yield.
Further, a method of reacting a diester of phosphorous acid, sulfur and methanol solution of ammonia was proposed. However, the method has a disadvantage that undesired dialkyl ester is obtained as a by-product due to an interesterification.